Study On Protection Of VSC-DC Distribution System

With the rapid development of renewable energy generations,voltage source converter(VSC)-based DC(Direct Current)distribution networks have been studied for their good performance in areas such as flexible controls,fewer conversion stages,excellent power quality,etc.Protection is the key to the safe and reliable operation of the VSC-DC distribution,and it is also one of the major problems.In the VSC-DC distribution,the fault current rises rapidly and the VSC is vulnerable to overcurrent.The protection must selectively identify the faulty line within a few milliseconds,which is a big challenge.The studies on protection are of major significance.Based on the research of time domain and frequency domain fault characteristics,this dissertation proposes two complementary protection methods and two fault location methods to different application scenarios.The main work and innovations in this dissertation are as follows:(1)Time domain and frequency domain fault characteristics.To the time domain characteristics,the components of fault quantities are analyzed and it is proved that the full current has a single direction during the fault process.To the frequency domain characteristics,the differences of frequency distribution between fault current and line distributed capacitance are revealed.The equivalent circuit of the AC feeding stage is derived and the existence of sixth harmonic component is proved.The fault characteristics lay the foundation for the study of protection principles and fault location methods.(2)Frequency distribution-based differential protection.The characteristics of differential current frequency band are presented and the frequency distribution-based differential protection is proposed.The discrete wavelet transform is adopted to calculate the energy of differential current frequency bands.The protection has absolute selectivity and can withstand a range of fault resistance and measurement noise.The protection algorithm can operate in less than 3ms and is not affected by the line capacitance.It is solved that the differential protection is vulnerable to the transient current of the line distributed capacitance.(3)Full current-based directional pilot protection.To complement the advantages of current differential protection,the differences of full current directions between internal and external faults are analyzed and the directional pilot protection based on full current is proposed.The protection operation criteria and setting methods are shown.Many factors influencing the protection are analyzed.It is proved that this method is not affected by the transient current of the line distributed capacitance.The proposed protection has a simple principle and can work during the whole process of faults.The communication need not be synchronized and the protection is not affected by the differences in current sensor characteristics.The protection algorithm can operate in less than 1 ms for internal faults under a range of measurement noise.The proposed protection can meet both the requirements on speed and selectivity.(4)Sixth harmonic-based fault location.Because the fault measurements for location methods are limited,the location methods need to utilize the fault transient information that contain multiple decaying components.Due to the influence of multiple decaying components,the Fourier algorithm is difficult to accurately extract the sixth harmonic component.Therefore,an improved Fourier algorithm is proposed.The redundant information is used to remove the effects of multiple decaying components,and the sixth harmonic component is accurately extracted.The accurate fault location is accomplished.Using local measurements,the method can locate fault with less than 2%error.(5)Improved R-L model fault location.Aiming at the VSC-DC distribution with limiting reactors,the time domain information-based fault location is proposed.With the time domain fault characteristics,the feasibility of the R-L model algorithm is analyzed.To decrease errors,a new variable acquisition method with the voltage of the limiting reactor is proposed.A new fault distance estimation formula are proposed to increase numerical stabilities.The time domain information-based fault location is proposed for the scenarios with limiting reactors.This method has theoretical validity during the whole fault process.This method can complete fault location with the sampling frequency of 1kHz and the location error remains within 1%.